The problem of controlling static charge is well known in the field of photography. The accumulation of charge on film or paper surfaces leads to the attraction of dirt, which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or “static marks” in the emulsion. The static problems have been aggravated by increase in the sensitivity of new emulsions, increase in coating machine speeds, and increase in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive “antistatic” layers into the film structure. Antistatic layers can be applied to one or to both sides of the film base as subbing layers either beneath or on the side opposite to the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivities. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility rather than ionic mobility and is independent of humidity. Antistatic layers that contain conjugated polymers, semiconductive metal halide salts, semiconductive metal oxide particles, etc., have been described previously. However, these antistatic layers typically contain a high volume percentage of electronically conducting materials, which are often expensive and impart unfavorable physical characteristics, such as color, increased brittleness and poor adhesion, to the antistatic layer.
Besides antistatic properties, an auxiliary layer in a photographic element maybe required to fulfill additional criteria depending on the application. For example for resin-coated photographic paper, the antistatic layer if present as an external backing layer should be able to receive prints (e.g., bar codes or other indicia containing useful information) typically administered by dot matrix printers and to retain these prints or markings as the paper undergoes processing. Most colloidal silica based antistatic backings without a polymeric binder provide poor post-processing backmark retention qualities for photographic paper.
Yet another important criterion for photographic paper is its spliceability. Heat splicing of photographic paper rolls is often carried out during printing operations and is expected to provide enough mechanical strength to resist peeling as the web goes at high speed through automatic photographic processors following complicated paths including many turns around transport and guide rollers which puts a great deal of stress on the paper. Heat splicing is typically carried out between the silver halide side of the paper and the antistatic backside of the paper. Poor splice strength can cause a number of problems including jamming of automatic processing equipment resulting in machine shut down. Antistatic backings with poor adhesion to the paper base and/or poor cohesive strength are likely to provide inadequate splice strength.
In general, poor adhesion of the antistatic coating onto the resin-coated paper base may be responsible for a number of problems during manufacturing, sensitizing and photofinishing. Poor adhesion or cohesion of the antistatic backing can lead to unacceptable dusting and track-off. The dust particles require periodic cleaning, which can hamper smooth, continuous running of any equipment, thereby affecting productivity. The dust particles can also cause physical defects during coating and sensitizing, generating unacceptable product quality and waste. A discontinuous antistatic layer, resulting from dusting, flaking, or other causes, may exhibit poor lateral conductivity, and may not provide necessary static protection. It can also allow leaching of calcium stearate from the paper support into the processing tanks causing build-up of stearate sludge. Flakes of the antistatic backing in the processing solution can form soft tar-like species which, even in extremely small amounts, can re-deposit as smudges on drier rollers eventually transferring to image areas of the photographic paper, creating unacceptable defects.
Although the prior art is replete with patents disclosing various antistatic backings for photographic paper (for example, U.S. Pat. Nos. 3,671,248; 4,547,445; 5,045,394; 5,156,707; 5,221,555; 5,232,824; 5,244,728; 5,318,886; 5,360,707; 5,405,907 and 5,466,536), not all of the aforesaid issues are fully addressed by these inventions. Also, some of the inventions of the prior art may alleviate one or more problems but may aggravate some others. For example, U.S. Pat. No. 3,525,621 teaches that antistatic properties can be given to an aqueous coating composition by practically any silica sol, but preferably a silica of large surface area of the order of 200–235 m2/g in combination with an alkylaryl polyether sulfonate. However, the high solubility of the alkylaryl polyether sulfonate in aqueous medium causes leaching during processing resulting in poor backmark retention of such antistatic layers. Similarly, U.S. Pat. No. 5,244,728 teaches a binder polymer consisting of an addition product of alkyl methacrylate, alkali metal salt and vinyl benzene which, when incorporated in an antistatic layer for photographic paper, substantially improves backmark retention characteristics but compromises spliceability and track-off characteristics, as demonstrated in U.S. Pat. No. 5,683,862. U.S. Pat. No. 5,466,536 teaches the use of a mixture of polymers and copolymers with specific acrylic acid content for good printabilty. However, the high acid number of these polymers make the antistatic layer (or debris thereof) vulnerable for softening in high pH developer solution, and can cause formation of soft tar-like species discussed herein above.
Moreover, backings developed for one type of polyolefin-coated paper may fail on a different type of polyolefin-coated paper. Therefore, although claims are generally made for both polyethylene and polypropylene coated photographic paper, a vast majority of patents in the art provide examples involving polyethylene coated photographic paper only, and the successful application of these teachings on polypropylene coated photographic paper is often, and even generally, not possible. In general, good adhesion of antistatic layers on a polypropylene surface is more difficult to achieve than on a polyethylene surface. For example, in U.S. Pat. No. 4,547,445 a layer containing gelatin and an inorganic pigment is claimed to have ink-retaining characteristics with good adhesion to polyethylene-coated photographic paper. But, as discussed in U.S. Pat. No. 5,853,965, such a gelatin containing layer is expected to fail adhesion on a biaxially oriented polypropylene-coated photographic paper. In fact, adhesion of auxiliary layers to polypropylene surfaces has become a key issue for reflective print media, as more and more products comprising such a surface are being disclosed in the patent literature and introduced to the market (vide, for example, U.S. Pat. Nos. 5,853,965; 5,866,282; and 5,874,205). Antistatic layers containing a styrene-maleic anhydride copolymer, colloidal silica and crosslinking compounds containing ethyleneimino groups and/or epoxy rings are disclosed in U.S. Pat. No. 4,266,016, allegedly for good antistatic characteristics and adhesion on both polyethylene and polypropylene surfaces. However, as demonstrated through comparative samples in U.S. Pat. No. 6,171,769, such antistatic layers provide neither the backmark retention characteristics nor the spliceability currently desired of photographic paper. U.S. Pat. No. 6,171,769, by itself, teaches of binder polymers with excellent adhesion to polypropylene surfaces. However, these binder polymers are not known to have any appreciable electrical conductivity, and, thus do not participate in antistatic function by themselves. Typically, for a given dry coverage of the antistatic layer, the higher the amount of binder polymer the better is the adhesion but poorer is the electrical conductivity of the layer
A vast majority of antistatic formulations designed for use in photographic reflective media are aqueous based coating compositions, utilizing salts for ionic conductivity, inorganic particles such as colloidal silica as fillers and latex polymers as binders. Although the salt is needed for electrical conductivity, its presence can adversely affect the dispersion of the latex and/or the colloidal filler, through charge screening. Such an adverse effect unacceptably increases the viscosity of the coating composition and/or its shelf life, rendering it impractical for robust manufacturing. A careful balance needs to be struck in the content of the various ingredients to maintain appropriate viscosity and yet achieve the physical properties, such as conductivity and adhesion to the substrate, required of the resultant antistatic layer. In this context, identification of a binder polymer, which adds to the electrical conductivity (and, therefore, requires less salt) as well as provides good adhesion to the support appears highly desirable.
Thus, it is clear that the known art does not fully meet the high demands and the diverse needs of the industry. Further innovation is needed. The present invention provides a composition suitable for use as an antistatic backing for photographic elements, particularly reflective print media, comprising at least one polyolefin layer, wherein the antistatic layer provides superior electrical conductivity, backmark retention, spliceability and dusting characteristics through improved adhesion to the support, formed out of robust coating compositions with controlled viscosity, fulfilling the stringent requirements of the industry.